Postgresql limit offset踩坑

版本：PG11
同事遇到对一个表分段查询但是没有使用order，导致不同数据段出现数据重复的情况。
翻阅官方文档后，发现PG的表是没有默认顺序的，使用limit offset必须使用order，否则会导致无法预料的顺序问题。

官方文档：https://www.postgresql.org/docs/11/queries-limit.html
原文摘抄：

LIMIT and OFFSET allow you to retrieve just a portion of the rows that are generated by the rest of the query:

SELECT select_list
FROM table_expression
[ ORDER BY ... ]
[ LIMIT { number | ALL } ] [ OFFSET number ]
If a limit count is given, no more than that many rows will be returned (but possibly less, if the query itself yields less rows). LIMIT ALL is the same as omitting the LIMIT clause.

OFFSET says to skip that many rows before beginning to return rows. OFFSET 0 is the same as omitting the OFFSET clause, and LIMIT NULL is the same as omitting the LIMIT clause. If both OFFSET and LIMIT appear, then OFFSET rows are skipped before starting to count the LIMIT rows that are returned.

When using LIMIT, it is important to use an ORDER BY clause that constrains the result rows into a unique order. Otherwise you will get an unpredictable subset of the query's rows. You might be asking for the tenth through twentieth rows, but tenth through twentieth in what ordering? The ordering is unknown, unless you specified ORDER BY.

The query optimizer takes LIMIT into account when generating query plans, so you are very likely to get different plans (yielding different row orders) depending on what you give for LIMIT and OFFSET. Thus, using different LIMIT/OFFSET values to select different subsets of a query result will give inconsistent results unless you enforce a predictable result ordering with ORDER BY. This is not a bug; it is an inherent consequence of the fact that SQL does not promise to deliver the results of a query in any particular order unless ORDER BY is used to constrain the order.

The rows skipped by an OFFSET clause still have to be computed inside the server; therefore a large OFFSET might be inefficient.`